Ordinarily an X-ray beam-generating device referred to as an X-ray tube comprises dual electrodes of an electrical circuit in an evacuated chamber or tube. One of the electrodes is an electron emitter cathode, which is positioned in the tube in spaced relationship to a target anode. Upon energization of the electrical circuit, the cathode generates a stream or beam of electrons directed towards the target anode. This acceleration is generated from a high voltage differential between the anode and cathode that may range from 60-600 kV, which is a function of the imaging application. The electron stream is appropriately focused as a thin beam of very high velocity electrons striking the target anode surface. The anode surface ordinarily comprises a predetermined material, for example, a refractory metal so that the kinetic energy of the striking electrons against the target material is converted to electromagnetic waves of very high frequency, i.e. X-rays, which proceed from the target to be collimated and focused for penetration into an object usually for internal examination purposes, for example, industrial inspection procedures, healthcare imaging and treatment, or security imaging applications, food processing industries. Imaging applications include, but are not limited to, Radiography, CT, X-ray Diffraction with Cone and Fan beam X-ray fields.
Well-known primary refractory and non-refractory metals for the anode target surface area exposed to the impinging electron beam include copper (Cu), Fe, Ag, Cr, Co, tungsten (W), molybdenum (Mo), and their alloys for X-ray generation. In addition, the high velocity beam of electrons impinging the target surface generates extremely high, localized temperatures in the target structure accompanied by high internal stresses leading to deterioration and breakdown of the target structure. As a consequence, it has become a practice to utilize a rotating anode target generally comprising a shaft supported disk-like structure, one side or face of which is exposed to the electron beam from the emitter cathode. By means of target rotation, the impinged region of the target is continuously changing to avoid localized heat concentration and stresses and to better distribute the heating effects throughout the structure. Heating remains a major problem in X-ray anode target structures. In a high speed rotating target, heating must be kept within certain proscribed limits to control potentially destructive thermal stresses particularly in composite target structures, as well as to protect low friction, solid lubricated, high precision bearings that support the target.
Only about 1.0% of the energy of the impinging electron beam is converted to X-rays with the remainder appearing as heat, which must be rapidly dissipated from the target essentially by means of heat radiation, convection and/or conduction. Accordingly, significant technological efforts are expended towards improving heat dissipation from X-ray anode target surfaces. For most rotating anode targets heat management must take place principally through radiation and a material with a high heat storage capacity. Stationary anode target body configurations or some complex rotating anode target configurations may be designed to have heat transfer primarily take place using conduction or convection from the target to the X-ray tube frame. Life of rotating X-ray targets is often gated by the complexities of rotation in a vacuum. Traditional X-ray target bearings are solid lubricated, which have relatively low life. Stationary targets do not have this life-limiting component, at the cost of lower performance.
Other rotation components, including, but not limited to, solid lubricated bearings, ferro-fluid seals, rotating vacuum envelope tubes, spiral-grooved liquid metal bearings, introduce manufacturing complexity and system cost.
What is needed is a high flux X-ray tube configuration that provides improved heat dissipation and includes components capable of maintaining an extended life, with a limited introduction of cost and manufacturing complexity.